Role in WW2
Main article: Soviet Union in World War II
The Soviet Union's participation in World War II began with the Battles of Khalkhin Gol against Japan, in Mongolia between May–August, 1939. Later that year, protected with the Molotov–Ribbentrop Pact, it invaded eastern Poland about three weeks after the Germans invaded the west of the country. During the next eleven months the Soviets occupied and annexed the Baltic states. The Soviet Union supported Germany in the war effort against Western Europe through the 1939 German–Soviet Commercial Agreement and larger 1940 German–Soviet Commercial Agreement with supplies of raw materials that were otherwise blocked by the British naval blockade. Following Finland's refusal of Soviet demands for military bases and a territorial swap, the Soviet Union invaded on 30 November 1939, in the Winter War. The Soviet Union also annexed Bessarabia, leading Romania to ally with Germany.
On June 22, 1941, Nazi Germany launched a massive surprise attack on the Soviet Union. Despite its colossal losses, Soviets alone were responsible for 80% of Germans WWII military casualties. Consequently, since most of the German forces were concentrated on the Eastern Front, USSR was able to push the Germans back once the Allies opened a second front in Europe.
After an initial devastating advance, the Wehrmacht suffered its first defeat in war at Moscow. The Germans and their allies tried in 1942 to advance southward, to the Caucasus. After six months of fighting, at Stalingrad they suffered a pivotal defeat. In late 1943, in the wake of Battle of Kursk, the Soviet Red Army gained the initiative with a series of major victories, culminating in the ultimate advance of Soviet forces into Eastern Europe during 1944 and over Germany in 1945, concluded by the Battle of Berlin. The war against the USSR inflicted loss of lives (both civilian and military), on a scale greater than any countries in the war. Following the end of the war in Europe and the American atomic bombing of Hiroshima, the USSR joined the war against Japan. The Soviet Union, as one of the main victors, gained one of the permanent seats in the United Nations Security Council. After the war, the Soviet sphere of influence was widened to cover most of Eastern Europe, formalized in the Warsaw Pact. The Soviet Union came to be considered one of the two superpowers of the Cold War.